Gems are rebuilt on gemdir input change

[samueldr]

See:

• https://github.com/fly-apps/rails-nix/tree/fix/gems-isolation

Since the gemdir is the src dir, any change will also cause a minor rebuild of the gems layer. Note that this only "re-assembles" the gem directory, no gems are being rebuilt. The actual cost in build time is negligible, but still present.

The solution listed here might not be correct, as it wouldn't include local directories used as gems. Though the general approach would be correct, e.g. use strictly only the inputs required for gems. Not sure how to best approach this.

[samueldr]

Oh, though it would make a needless layer upload for the gems layer. This is something the CAS (content-adressed store) would help solve, but this is not a solution for now.

[samueldr]

Discussed quickly, we will work under the assumption that no local gems are present, so it's fine to only work on Gemfile+Gemfile.lock

[jsierles]

Fixed in fly-apps/nix-base#3